There are many situations in which it is desired to determine the position of an object. A typical situation is that in which the object is a movable member of a mechanism, wherein it is desired to determine when the member has moved to a certain position. Quite often, such position determination is made by a proximity sensor that includes a target affixed or linked to the movable member and a proximity switch mounted at a reference position. The target and the proximity switch are constructed and arranged so that the proximity switch provides a switch actuation whenever the target is at a predetermined distance from the proximity switch.
It has been proposed to use a fiber optic sensor to provide position determination. Such a fiber optic sensor would include at least one optical fiber extending from a sensor circuit to the reference position, and a target that is capable of illumination by the optical fiber and that is either formed in, affixed to, or linked to the movable member. The target would be provided with target areas whose light reflectivity contrasts with that of the target background and which are appropriately located on the target so as to correspond to discrete positions of the movable member relative to the reference position. The sensor circuit may then transmit light to the target through the optical fiber and provide position determination by analyzing that portion of the transmitted light that is reflected by the target areas and their background to the sensor circuit through the optical fiber.
One of the requirements for a fiber optic sensor particularly adapted for position determination is that each target area, whether it be a "light" area of relatively high light reflectivity on a "dark" background of relatively low light reflectivity, or a dark area on a light background, must be well-defined in dimension and must have a light reflectivity that is and remains in sharp contrast to that of the target background. The present invention is specifically directed to meeting this requirement in the case where the target area is substantially nonreflective and the target background is highly reflective, and also meets other requirements of specific position determination applications as will be apparent from the ensuing discussion.